This invention relates to a hearing aid comprising a controllable oscillator and a control circuit coupled thereto.
Such a hearing aid is disclosed in DE-C 4 221 304. Digital hearing aids require an oscillator to generate a clock signal. To obtain properly functioning hearing aids, the frequency of this clock signal must be determined in a stable and accurate manner. In general, such a stable and accurate clock signal can be obtained in a simple manner by using a crystal oscillator. However, crystal oscillators are not often used in hearing aids because of the dimensions of the crystals required by these oscillators. Instead, hearing aids are customarily provided with a controllable oscillator. In such hearing aids, the frequency of the clock signal generated by the controllable oscillator, which frequency is governed, inter alia, by voltage and temperature variations, can be regulated.
The hearing aid known from the above-mentioned German patent specification comprises a controllable oscillator and a control circuit which is coupled thereto. This control circuit comprises a memory in which a number of data words are stored. By manually selecting one specific data word from this memory, the frequency of the signal generated by the oscillator is set via a number of capacitors. However, in the known hearing aid, a fine adjustment of the oscillator frequency is not readily possible.